When images are printed by printing devices, various defects and irregularities can appear in the printed image, for example dot placement error, lack of fidelity in reproduction of colours, and so on. There are also various causes of printing defects, for example ink-drop weight variability, misalignment of printheads, and so on. In multi-pass printing the location where an irregularity occurs on each pass may become randomized, so that irregularities are reduced or, at least, they are less visible in the final printed image. In one-pass printing this randomizing of the positions of irregularities does not occur.
Some printing devices include components designed to detect and compensate for printing irregularities, for example by printing a calibration pattern, by automatically detecting and analysing the printed calibration pattern and then performing some compensation operation based on the result of the analysis. In some cases the component used for detecting the printed calibration pattern has a limited resolution and, in particular, cannot accurately detect irregularities that are smaller than a certain size. For example, this may be the case for the densitometers used in some printing devices. However, the human eye may still be able to detect these small irregularities.
In some cases it may be assumed that users who employ a one-pass printing mode will be prepared to tolerate a reduced quality printed image, so no extra measures will be taken. The underlying assumption would be that the user can employ a multi-pass print mode if a higher quality printed image is desired. However, an alternative approach would be to improve the detection component that is used in the printing device to detect the calibration pattern. This could be done, for example, by adding a further sensor or scanning element, or by using a higher-performance detection element. Of course, use of an additional component or use of a higher-performance detector would be expected to increase the cost of the printing device.
Another alternative approach would be to make a detailed measurement of the printing irregularities produced by a printing device at the stage where the device is being manufactured, and to build into the printing device a pre-calculated correction or compensation. However, the use of a predetermined correction may not be adequate to compensate for the printing irregularity in the case where the printing error varies in a dynamic manner, for example based on environmental factors, printing speed, and so on.
Page wide array printing devices (PWA printing devices) have come into use and can print simultaneously over the whole width of a substrate. It is common for PWA printing devices to implement one-pass printing. When a PWA printing device uses a scanning element (e.g. a densitometer) that is not capable of accurately detecting small irregularities produced during one-pass printing but which are still visible to the human eye, the user may consider that the quality of the printed image to be inadequate.